Many miles of existing highways comprise asphalt or comparable material in combination with various aggregates such as gravel, crushed stone, and sand. In addition, numerous additional miles are built each year in which asphalt serves as a major component. The continued widespread use of such material, however, depends substantially upon cost, durability of the surfaces, and the frequency and nature of required preventative maintenance in response to constant weather and vehicular-induced damage.
Generally speaking, when repair or resurfacing is carried out, liquified asphalt or asphalt emulsions are applied in one form or another as filler for road cracks, as a waterproof underlayer between old and proposed new paving surfaces, and as the extenal surfacing material itself. For each purpose, however, the amount and fineness of aggregate, the nature and amount of curing and thickening agent and concentration of fiber and other additives can vary widely.
The oiling of asphalt-based surfaces has long standing use for preventative maintenance purposes. Such treatment, however, has many associated limitations and problems, particularly when accompanied by loss in skid resistance and limited lifetime, and where available manpower and budgets are not sufficient to assure consistent adherence to a regular maintenance schedule. The answer, particularly in the later case, is the utilization of a surface having greater durability and flex, which requires less maintenance. Also very useful and desirable, is a durable paving and sealing composition which is applicable under a wide range of temperature conditions, particularly lower temperatures, and which can be laid down and cured within a reasonably short period of time.
Such characteristics are very important when the treatment requires a slurry seal. Here substantial aggregate is utilized in combination with an emulsion and applied onto the damaged surface by using dry box or similar art-recognized applicator devices. Such coating is customarily applied to a depth of about 1/4", and at a concentration of about 10-25 lbs/yd.sup.2, to form an essentially new surface.
While the above treatment avoids some problems, it is much more expensive than oiling. Moreover, substantial room remains for improvement with respect to durability, lay down characteristics, and curing time. Such is particularly the case when durability is dependent upon retention of hydrophobic properties and resistance to low temperature-induced brittleness.
Chip seal treatment is also frequently used to upgrade damaged surfaces. In such treatment, a relatively thick seal or membrane of hot asphalt is laid down, followed by distribution of aggregate thereon with pressing and rolling to smooth the surface and hold the aggregate firmly in the seal or membrane. Here again, however, there remains a need for improved sealing properties, flexibility, and durability, commensurate with the use of regular paving equipment without unduly lengthening the curing time.
Flex and durability have been substantially improved with respect to paving and sealing compositions by including reinforcing fiber materials, such as fiberglass, asbestos, and particularly synthetic polymeric staple fiber, such as polyolefins, in paving compositions.
A preferred synthetic fiber material for such purposes, is found to be polypropylene staple fiber, because of a high degree of compatibility with asphalt-, asphalt/rubber-, and asphalt/acrylic copolymer-based emulsion components. Such material is conveniently used in amounts of 0.25 to about 10 weight percent reinforcing fiber by weight of base component or emulsion solids, and preferably about 0.25-6.0 weight percent, depending upon intended use.
Addition of synthetic hydrophobic fiber material such as polypropylene also causes some serious problems, however, since it adversely affects pumpability and workability characteristics of the composition, causing fiber clumping during conventional smoothing operations. Moreover, such fiber-reinforced compositions require at least a 10.degree. higher temperature than the usual 130.degree.-145.degree. C. application range, to obtain a workable lay down viscosity. Such heating is undesirable, not only because of higher energy costs, but because of possible temperature-induced degradation of the fibers, and because of the narrowed "temperature window" through which lay down can be successfully carried out under exposed conditions.
Associated with the presence of rubber components such as latex, is the problem of reasonable curing time, and a need for adequate storage life to meet planned or unexpected delay between preparation and lay down of paving or sealing compositions. Such goals are seemingly incompatible, particularly with respect to fiber-containing asphalt/rubber and asphalt/acrylic copolymer-containing compositions for crack seal and slurry seal purposes.
It is an object of the present invention to optimize seemingly inconsistent paving composition properties, particularly with respect to fiber-reinforced compositions.
It is a further object of the present invention to improve control over workability, cure time and durability of paving and sealing compositions containing reinforcing fiber material.
It is a still further object of the present invention to improve resistance to fiber clumping and to increase the internal adhesion of asphalt-, asphalt/rubber-, rubber-, acrylic copolymer-, and asphalt/acrylic copolymer-containing compositions to permit full and effective use of reinforcing fiber material, particularly polymeric polyolefin fibers such as polypropylene fiber.
It is an additional object of the present invention to develop a method for improving workability, cure time and durability of paving and sealing compositions containing polypropylene fiber material.